As known in the art, the CD (compact disc) system currently employed as a main-current acoustic apparatus is of such a type that pits are formed in a 12 cm-diameter disc in a manner to correspond to digital PCM (pulse code modulation) data and reproduction is performed by, while rotating the disc in a drive rotation in a CLV (constant linear velocity) system, linearly tracking the data from an inner periphery side to an outer periphery side on the disc by a semiconductor laser and an optical pickup built in a photoelectric conversion element.
Audio playback musical tone data and sub-code data P, Q, R--W defined for control/display are recorded in the disc. Of the aforementioned data, the sub-code Q is also called as address data and represents, in a program area containing disc's musical tone data, a program number (TNO) of recorded musical tone data, a phrase number (INDEX), a lapse time (TIME) for each program, a total time (ATIME) lapsing from a start position of the program area, etc.
Further, the aforementioned sub-code data Q represents, in a lead-in area located on the inner periphery side of the program area, a start address of each program as TOC (table of contents) data. That is, the sub-code data Q is recorded for implementing a search operation, accurately at high speeds, for selective reproduction of playback data from vast information recorded in the disc to enable a stereo reproduction to be performed for about one hour and for grasping the state of a reproduction on the disc.
The CD system has initially been developed so as to record and reproduce musical data. In recent years, with attention paid to such a vast recording capacity, a CD-ROM (read only memory) system is determined to be used as a read-only data recording medium for the disc through the use of a musical data recording area of the disc as a digital information recording area. The CD-ROM system is of such a type as to record and reproduce digital information on the disc without varying a recording/reproduction format for musical data reproduction in the CD system and to do so by adding a new format thereto.
In the conventional digital data processing system using the CD-ROM system, however, if a clock frequency as a standard of a data processing speed is shifted on the host processing section side and on the disc reproduction section side, then the buffer memory on the host processing section side is placed in an over- or an underflowed state, failing to perform a normal data reproduction and hence involving a problem.
The present invention has been achieved, taking the aforementioned situation into consideration, and the object of the present invention is to provide a highly improved digital data processing apparatus which, even if there is any shift in the frequency of a clock serving as a reference for a data processing speed on the host processing section side and on the disc reproduction section side, prevents the buffer memory on the host processing section side from being placed in an over- or an underflowed state and can perform a normal data reproduction.